Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
toss-up is a noun meaning "an unpredictable phenomenon". It carries one recorded sense.
an unpredictable phenomenon
“it's a toss-up whether he will win or lose”
Definitions, synonyms and semantic relations for toss-up come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.